The present invention relates to a method for preparing 4-hydroxy-1,2,2,6,6-pentamethylpiperidine (hereinafter, sometimes abbreviated to "N-methyl-TAAM") through methylation of 4-hydroxy-2,2,6,6-tetramethylpiperidine (known as triacetonalkamine; hereinafter, sometimes abbreviated to "TAAM").
N-methyl-TAAM is useful as a starting material for synthesizing sterically hindered piperidine compounds used as photostabilizers for plastics.
N-methyl-TAAM is prepared through methylation of TAAM, for example, by N-methylation of TAAM using formaldehyde/formic acid, which is known as the Eschweiler-Clarke reaction. This reaction is believed to proceed by the mechanism described below, with the formic acid being considered to play the role of reducing the enamine intermediate. ##STR1##
As references concerning this reaction, U.S. Pat. No. 3,974,127 and Japanese Unexamined Patent Publication No. 2-212479 are given.
U.S. Pat. No. 3,974,127 teaches that N-methylation of TAAM is performed by adding 18 g of 37% aqueous formaldehyde and 4 ml of formic acid containing 1% of water to 18 g of TAAM; thereafter, sodium hydroxide is added to the reaction mixture; and the reaction product is extracted with an ether.
Japanese Unexamined Patent Publication No. 2-212479 teaches that formaldehyde or paraformaldehyde which are at least 20 mole % in excess over TAAM and a substantially stoichiometric amount of formic acid are reacted and that a base is added to the reaction mixture before distillation of the reaction product.